Polarity control for remote plasma

ABSTRACT

Methods of controlling the polarity of capacitive plasma power applied to a remote plasma are described. Rather than applying a plasma power which involves both a positive and negative voltage swings equally, a capacitive plasma power is applied which favors either positive or negative voltage swings in order to select desirable process attributes. For example, the plasma power may be formed by applying a unipolar oscillating voltage between an electrode and a perforated plate. The unipolar oscillating voltage may have only positive or only negative voltages between the electrode and the perforated plate. The unipolar oscillating voltage may cross electrical ground in some portion of its oscillating voltage.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of U.S. Prov. Pat. App. No.61/911,791 filed Dec. 4, 2013, and titled “POLARITY CONTROL FOR REMOTEPLASMA” by Cho et al., which is hereby incorporated herein in itsentirety by reference for all purposes.

FIELD

Embodiments of the invention relate to polarity control using a remoteplasma.

BACKGROUND

Integrated circuits are made possible by processes which produceintricately patterned material layers on substrate surfaces. Producingpatterned material on a substrate requires controlled methods forremoval of exposed material. Chemical etching is used for a variety ofpurposes including transferring a pattern in photoresist into underlyinglayers, thinning layers or thinning lateral dimensions of featuresalready present on the surface. Often it is desirable to have an etchprocess which etches one material faster than another helping e.g. apattern transfer process proceed. Such an etch process is said to beselective of the first material. As a result of the diversity ofmaterials, circuits and processes, etch processes have been developedthat selectively remove one or more of a broad range of materials.However, there are few options for selectively etching silicon using gasphase reactants.

Dry etch processes are often desirable for selectively removing materialfrom semiconductor substrates. The desirability stems from the abilityto gently remove material from miniature structures with minimalphysical disturbance. Dry etch processes also allow the etch rate to beabruptly stopped by removing the gas phase reagents. Some dry-etchprocesses involve the exposure of a substrate to remote plasmaby-products formed from one or more precursors. For example, remoteplasma generation of nitrogen trifluoride in combination with ionsuppression techniques enables silicon to be selectively removed from apatterned substrate when the plasma effluents are flowed into thesubstrate processing region.

Methods are needed to improve process control for remote plasmaprocesses.

SUMMARY

Methods of controlling the polarity of capacitive plasma power appliedto a remote plasma are described. Rather than applying a plasma powerwhich involves both a positive and negative voltage swings equally, acapacitive plasma power is applied which favors either positive ornegative voltage swings in order to select desirable process attributes.For example, the plasma power may be formed by applying a unipolaroscillating voltage between an electrode and a perforated plate. Theunipolar oscillating voltage may have only positive or only negativevoltages between the electrode and the perforated plate. The unipolaroscillating voltage may be formed by offsetting an input oscillatingvoltage with a DC offset voltage, or it may be formed by rectifying aninput oscillating voltage using a diode or diode bridge or it may beformed from an input DC voltage processed using switching devices toform a unipolar square-wave voltage. The unipolar oscillating voltagemay have mostly positive or mostly negative voltages between theelectrode and the perforated plate. The remote plasma may be used toexcite a fluorine-containing precursor whose effluents are passed into asubstrate processing region to selectively etch a patterned substrate.

Embodiments of the invention include methods of processing a substrate.The methods include transferring the substrate into a substrateprocessing region of a substrate processing chamber. The methods furtherinclude applying a unipolar oscillating voltage between an electrode anda perforated plate. The unipolar oscillating voltage, on average, biasesthe perforated plate at a positive voltage relative to the electrode.The methods further include forming a remote plasma between theelectrode and the perforated plate to form plasma effluents. The methodsfurther include flowing the plasma effluents into the substrateprocessing region housing the substrate. The plasma effluents flow intothe substrate processing region through perforations in the perforatedplate. The methods further include reacting the plasma effluents withthe substrate.

Embodiments of the invention include methods of etching a substrate. Themethods include transferring the substrate into a substrate processingregion of a substrate processing chamber. The methods further includeapplying a unipolar oscillating voltage between an electrode and aperforated plate. The unipolar oscillating voltage negatively biases theelectrode relative to the perforated plate. The methods further includeflowing a halogen-containing precursor into a remote plasma between theelectrode and the perforated plate while forming a remote plasma to formplasma effluents. The methods further include flowing the plasmaeffluents into the substrate processing region housing the substrate.The plasma effluents flow into the substrate processing region throughperforations in the perforated plate. The methods further includeetching the substrate with the plasma effluents.

Embodiments of the invention include methods of etching a patternedsubstrate. The methods include transferring the patterned substrate intoa substrate processing region of a substrate processing chamber. Themethods further include sending a DC voltage through a full bridge offour switching devices to form a unipolar oscillating voltage. Themethods further include applying the unipolar oscillating voltagebetween a concave electrode and a flat nickel-plated perforated plate.The unipolar oscillating voltage biases the concave electrode at anegative voltage relative to the flat nickel-plated perforated plate.The methods further include flowing a fluorine-containing precursor intoa remote plasma between the concave electrode and the flat nickel-platedperforated plate while forming a remote plasma to form plasma effluents.The methods further include flowing the plasma effluents into thesubstrate processing region housing the patterned substrate. The plasmaeffluents flow into the substrate processing region through perforationsin the perforated plate. The methods further include selectively etchingthe patterned substrate with the plasma effluents.

Additional embodiments and features are set forth in part in thedescription that follows, and in part will become apparent to thoseskilled in the art upon examination of the specification or may belearned by the practice of the disclosed embodiments. The features andadvantages of the disclosed embodiments may be realized and attained bymeans of the instrumentalities, combinations, and methods described inthe specification.

DESCRIPTION OF THE DRAWINGS

A further understanding of the nature and advantages of the disclosedembodiments may be realized by reference to the remaining portions ofthe specification and the drawings.

FIG. 1 is a graphic showing a bipolar oscillating voltage, a concaveelectrode, and resulting plasma location according to embodiments.

FIG. 2 is a graphic showing a unipolar oscillating voltage and resultingplasma according to embodiments.

FIG. 3 is a flow chart of a substrate processing sequence according toembodiments.

FIG. 4A shows a substrate processing chamber according to embodiments ofthe invention.

FIG. 4B shows a showerhead of a substrate processing chamber accordingto embodiments of the invention.

FIG. 5 shows a substrate processing system according to embodiments ofthe invention.

In the appended figures, similar components and/or features may have thesame reference label. Further, various components of the same type maybe distinguished by following the reference label by a dash and a secondlabel that distinguishes among the similar components. If only the firstreference label is used in the specification, the description isapplicable to any one of the similar components having the same firstreference label irrespective of the second reference label.

DETAILED DESCRIPTION

Methods of controlling the polarity of capacitive plasma power appliedto a remote plasma are described. Rather than applying a plasma powerwhich involves both a positive and negative voltage swings equally, acapacitive plasma power is applied which favors either positive ornegative voltage swings in order to select desirable process attributes.For example, the plasma power may be formed by applying a unipolaroscillating voltage between an electrode and a perforated plate. Theunipolar oscillating voltage may have only positive or only negativevoltages between the electrode and the perforated plate. The unipolaroscillating voltage may be formed by offsetting an input oscillatingvoltage with a DC offset voltage, or it may be formed by rectifying aninput oscillating voltage using a diode or diode bridge or it may beformed from an input DC voltage processed using switching devices toform a unipolar square-wave voltage. A unipolar oscillating voltage maybe generally positive or negative while having a much smaller amount ofnegative or positive voltages, respectively. In other words, a unipolaroscillating voltage may cross electrical ground in some portion of theoscillating voltage. The remote plasma may be used to excite afluorine-containing precursor whose effluents are passed into asubstrate processing region to selectively etch a patterned substrate.

Reference is now made to FIG. 1 which is a graphic showing a bipolaroscillating voltage used to drive a concave electrode and the plasmalocations which have been found to occur during the positive andnegative swings embodiments. A bipolar sinusoidal voltage is shown andis symmetric around electrical ground. The bipolar sinusoidal voltagerepresents a sinusoidally varying voltage (varying in time) applied toan electrode relative to a grounded perforated plate. The electrode isnot planar and the perforated plate is planar according to embodiments.As shown, the plasma has been found to change positions within theremote plasma region between the non-planar electrode and the perforatedplate. The bipolar sinusoidal voltage initially swings positive suchthat the non-planar electrode is positively biased relative to thegrounded perforated plate. During this time, the plasma is located closeto the perforated plate. When the sinusoidal voltage swings negative,the non-planar electrode is negatively biased relative to the groundedperforated plate and the plasma switches to a position farther away fromthe perforated plate. Other shapes of bipolar oscillating voltages maybe used to drive the concave electrode and the locations of the twoplasma excitations would be similar.

The non-planar electrode may be referred to as a concave electrode (or ahollow cathode) when the interior surface is cup-shaped as shown. Aconcave electrode forms a larger volume near the center of the remoteplasma region. A perpendicular distance from the perforated plate to aconcave electrode may be greater near the center of the perforated platethan a perpendicular distance from the perforated plate to the electrodenear the edge. The remote plasma on the left of FIG. 1 may be referredto as “glow discharge mode” and the remote plasma on the right of FIG. 1may be referred to as “hollow cathode mode.” The hollow cathode mode hasbeen found to be desirable in embodiments. The concave electrode isnegatively biased during the hollow cathode mode which directs ions awayfrom the perforated plate and may reduce sputtering effects. Theperforated plate may be coated with nickel in embodiments. A substrateis disposed below the perforated plate during processing and substrateshave been found to possess more micro-contamination (in the form ofelevated particle count) in glow discharge mode relative to hollowcathode mode. The benefit may also arise from the location of the plasmarelative to the perforated plate and may result from the presence of thenickel coating.

FIG. 2 is a graphic showing a unipolar oscillating voltage according toembodiments. A bipolar sinusoidal voltage is processed to switch thesign of the positive swing, in embodiments, so that the resultingunipolar oscillating voltage possesses two negative swings per period ofthe original bipolar sinusoidal voltage. The period of the unipolaroscillating voltage is half the period of the original bipolarsinusoidal voltage. The unipolar oscillating voltage is still arepetitive oscillating voltage but may not be referred to as asinusoidal oscillating voltage anymore. The unipolar oscillating voltageis applied to the concave electrode and the plasma predominantly formsin the same portion of the remote plasma region during every peak of theunipolar oscillating voltage and not just half of the peaks. The bipolarsinusoidal voltage may be processed by passing the bipolar sinusoidalvoltage through a diode bridge formed from any appropriate type of fourdiodes. High voltage bipolar sinusoidal voltages may be processed usingIGBT (insulated gate bipolar transistor) diodes and low voltage bipolarsinusoidal voltages (less than 1-2 kV) may be processed using FET-stylediodes. A full bridge rectifying circuit would eliminate the ability ofthe bipolar oscillating voltage to excite a plasma so only a portion ofa rectification operation is performed on the bipolar sinusoidal voltageas shown in FIG. 2. Alternatively, the bipolar sinusoidal voltage may beprocessed with a more simplistic circuit (involving e.g. a single diode)to create a unipolar oscillating voltage with one negative swing foreach period of the original bipolar oscillating voltage. In this case,the unipolar oscillating voltage would possess the same period as thebipolar sinusoidal voltage. Alternatively, the unipolar oscillatingvoltage may be formed by passing a DC voltage through a full bridgeformed from any appropriate type of four switching devices.

In order to better understand and appreciate the invention, reference isnow made to FIG. 3 which is a flow chart of a substrate processingmethod 300 according to embodiments. A substrate (which may bepatterned) is transferred into a substrate processing region inoperation 310. A DC voltage may then be processed to form a “squarewave” for the unipolar oscillating voltage (operation 320) of negativesign. Alternatively, a sinusoidal voltage may be processed using a diodebridge (operation 320) to form a unipolar oscillating voltage which mayalways be negative relative to the potential of a nickel-platedperforated plate. The nickel-plated perforated plate is connected toelectrical ground in substrate processing sequence 300 but may be heldat another potential in embodiments. The unipolar oscillating voltage isapplied to a concave electrode relative to the nickel-plated perforatedplate (operation 325).

Nitrogen trifluoride is flowed into a remote plasma region and a remoteplasma power is applied (operation 330) to form plasma effluents. Theremote plasma region is separated from the substrate processing regionby the nickel-plated perforated plate. The plasma effluents are flowedinto the substrate processing region (operation 335). Other sources offluorine may be used to augment or replace the nitrogen trifluoride. Ingeneral, a fluorine-containing precursor may be flowed into the remoteplasma region and the fluorine-containing precursor may include one ormore of atomic fluorine, diatomic fluorine, boron trifluoride, chlorinetrifluoride, nitrogen trifluoride, perfluorinated hydrocarbons, sulfurhexafluoride and xenon difluoride. The plasma effluents formed in theremote plasma region are then flowed into the substrate processingregion (operation 335) through perforations in the nickel-platedperforated plate which separates the remote plasma region and thesubstrate processing region. One material on the patterned substrate isselectively etched relative to another (operation 345). The reactivechemical species and any process effluents are removed from thesubstrate processing region and then the substrate is removed from thesubstrate processing region.

The remote plasma region is located within a compartment within thesubstrate processing chamber between the electrode and the perforatedplate. The remote plasma region may is fluidly coupled to the substrateprocessing region by way of perforations in perforated plate. Thehardware just described (and elaborated on in the equipment section) mayalso be used in all processes discussed herein. The perforated plate maybe the showerhead described herein or it may be the ion suppressionelement according to embodiments. The perforated plate may also describethe combination of an ion suppression element and a showerhead.

The selective etching operation 345 may remove one material faster thananother. For example, silicon on the substrate may be selectively etchedsuch that silicon is removed more rapidly than a variety of othermaterials. The etch selectivity (silicon:silicon oxide orsilicon:silicon nitride) may also be greater than or about 70:1, greaterthan or about 100:1, greater than or about 150:1, greater than or about200:1, greater than or about 250:1 or greater than or about 300:1according to embodiments. Regions of exposed tungsten or titaniumnitride may be present, in embodiments, on the patterned substrate andmay be referred to as exposed metallic regions. The etch selectivity(silicon:exposed metallic region) may be greater than or about 100:1,greater than or about 150:1, greater than or about 200:1, greater thanor about 250:1, greater than or about 500:1, greater than or about1000:1, greater than or about 2000:1 or greater than or about 3000:1according to embodiments. The flow rate ranges now given forfluorine-containing precursors apply to flowing operation 330 (as wellas all flowing operations described herein). In embodiments, thefluorine-containing precursor (e.g. NF₃) is supplied at a flow rate ofbetween about 5 sccm and about 500 sccm, between about 10 sccm and about300 sccm, between about 25 sccm and about 200 sccm, between about 50sccm and about 150 sccm or between about 75 sccm and about 125 sccm.

The method also includes applying energy to the fluorine-containingprecursor in the remote plasma region to generate the plasma effluentsin operation 330 (as well as all remote plasmas described herein). Theplasma may be generated using low frequency excitations applied usingcapacitively-coupled power according to embodiments. The remote plasmasource power may be between about 10 watts and about 3000 watts, betweenabout 20 watts and about 2000 watts, between about 30 watts and about1000 watts in embodiments. The low frequency excitation may be a drivingvoltage in the form of a square wave, triangle wave or sinusoidalvoltage but may also be a variety of repetitive voltages (aka unipolaroscillating voltages according to embodiments. The unipolar oscillatingvoltage may have a unipolar oscillating frequency of less than or about1,000 kHz, less than or about 500 kHz, less than or about 300 kHz orbetween 1 kHz and 200 kHz according to embodiments. The peak-to-peakamplitude of the unipolar oscillating voltage may greater than 300volts, greater than 500 volts or greater than 800 volts in embodiments.

Generally speaking, a unipolar oscillating voltage is applied betweenthe non-planar electrode and the perforated plate. The differentialvoltage applied in this way will be referred to as the unipolaroscillating voltage. For example, the unipolar oscillating voltage maybe applied to the non-planar electrode while the perforated plate is atelectrical ground. The unipolar oscillating voltage may be applied tothe perforated plate while the non-planar electrode is at electricalground according to embodiments. In embodiments, neither the perforatedplate nor the non-planar electrode is at ground but the unipolaroscillating voltage is still applied as a differential voltage betweenthe perforated plate and the non-planar electrode.

The unipolar oscillating voltage may be formed using a variety oftechniques. A DC input voltage may be chopped into a unipolaroscillating voltage which is a “square wave” aside from slew rateeffects. An input voltage may be processed to create the unipolaroscillating voltage used to create the plasma. The unipolar oscillatingvoltage is applied between the non-planar electrode and the perforatedplate. The input voltage may be processed using a diode or diode bridge,as described previously, to form the unipolar oscillating voltage. Theinput voltage may be a sinusoidally-varying function in time centeredabout the potential of the perforated plate or ground, in embodiments.The input voltage may be shifted with a DC offset voltage such that theunipolar oscillating voltage is no longer centered around the potentialof the perforated plate. Regardless of the processing technique used,the electrical potential of the non-planar electrode may be greater thanthe electrical potential of the perforated plate more than 85% of thetime, more than 90% of the time, more than 95% of the time or all thetime according to embodiments. Alternatively, the electrical potentialof the non-planar electrode may be less than the electrical potential ofthe perforated plate more than 85% of the time, more than 90% of thetime, more than 95% of the time or all the time according toembodiments. Alternatively, the electrical potential of the non-planarelectrode may have a negative peak differential voltage whose magnitudeexceeds its positive peak voltage of the perforated plate (with eachpeak measured from the potential of the perforated plate) by amultiplicative factor of 5, a multiplicative factor of 10 or amultiplicative factor of 20 in embodiments. Analogously, the electricalpotential of the non-planar electrode may have a positive peakdifferential voltage whose magnitude exceeds its negative peak voltageof the perforated plate (with each peak measured from the potential ofthe perforated plate) by a multiplicative factor of 5, a multiplicativefactor of 10 or a multiplicative factor of 20 according to embodiments.

In all embodiments described herein which use a remote plasma, the term“plasma-free” may be used to describe the substrate processing regionduring application of no or essentially no plasma power. A plasma-freesubstrate processing region may be used during substrate processingmethod 300 in embodiments.

The temperature of the substrate for all embodiments described hereinmay be greater than 0° C. during the etch process. The substratetemperature may be greater than or about −20° C. and less than or about300° C. The pressure in the substrate processing region may be similarto the pressure in the remote plasma region during substrate processingmethod 300. The pressure within the substrate processing region may bebelow or about 10 Torr, below or about 5 Torr, below or about 3 Torr,below or about 2 Torr, below or about 1 Torr or below or about 750 mTorraccording to embodiments. In order to ensure adequate etch rate, thepressure may be above or about 0.05 Torr, above or about 0.1 Torr, aboveor about 0.2 Torr or above or about 0.4 Torr in embodiments. Any of theupper limits on pressure may be combined with lower limits according toembodiments.

In each remote plasmas described herein, the flows of the precursorsinto the remote plasma region may further include one or more relativelyinert gases such as He, N₂, Ar. The inert gas can be used to improveplasma stability, ease plasma initiation, and improve processuniformity. Argon is helpful, as an additive, to promote the formationof a stable plasma. Process uniformity is generally increased whenhelium is included. These additives are present in embodimentsthroughout this specification. Flow rates and ratios of the differentgases may be used to control etch rates and etch selectivity.

The non-planar electrode may be a variety of shapes and may be a concaveelectrode in disclosed embodiments. A concave electrode may include aportion shaped like a cone, a surface of revolution formed using aparabola, a surface of revolution formed using a circle or a surface ofrevolution using any generatrix according to embodiments.

In embodiments, an ion suppressor as described in the exemplaryequipment section may be used to provide radical and/or neutral speciesfor selectively etching substrates. The ion suppressor may also bereferred to as an ion suppression element. In embodiments, for example,the ion suppressor is used to filter fluorine-containing plasmaeffluents to selectively etch silicon. The ion suppressor may beincluded in each exemplary process described herein. Using the plasmaeffluents, an etch rate selectivity of a selected material to a widevariety of materials may be achieved.

The ion suppressor may be used to provide a reactive gas having a higherconcentration of radicals than ions. The ion suppressor functions todramatically reduce or substantially eliminate ionically charged speciestraveling from the plasma generation region to the substrate. Theelectron temperature may be measured using a Langmuir probe in thesubstrate processing region during excitation of a plasma in the remoteplasma region on the other side of the ion suppressor. In embodiments,the electron temperature may be less than 0.5 eV, less than 0.45 eV,less than 0.4 eV, or less than 0.35 eV. These extremely low values forthe electron temperature are enabled by the presence of the showerheadand/or the ion suppressor positioned between the substrate processingregion and the remote plasma region. Uncharged neutral and radicalspecies may pass through the openings in the ion suppressor to react atthe substrate. Because most of the charged particles of a plasma arefiltered or removed by the ion suppressor, the substrate is notnecessarily biased during the etch process. Such a process usingradicals and other neutral species can reduce plasma damage compared toconventional plasma etch processes that include sputtering andbombardment. The ion suppressor helps control the concentration of ionicspecies in the reaction region at a level that assists the process.Embodiments of the present invention are also advantageous overconventional wet etch processes where surface tension of liquids cancause bending and peeling of small features.

FIG. 3 is a plot of silicon etch rates 300 without a preventativemaintenance procedure and following a preventative maintenance procedureaccording to embodiments. The plot of silicon etch rates 300 includesetch rate measurements performed over a sequence of 900 wafers (anexample of substrates). Data 310 are included showing changes to etchrate of silicon over time while processing a sequence of wafers withoutany treatment operation prior to processing the wafers. The etch ratecan be shown drifting upward as, presumably, the interior surfacesbordering the substrate processing region evolve over time. Data 320 arealso included which show a relatively stable silicon etch rate followingtreatment operation 125 (using a fluorine-containing precursor). Data330 are also included which show a relatively silicon etch ratefollowing treatment 225 (using a hydrogen-containing precursor). Thedata following fluorine treatment operation 125 and the data followinghydrogen treatment operation 225 are both indicative of a stable siliconetch rate which are each desirable in a manufacturing environment. Notethat the magnitude of the etch rate following each of the twopreventative maintenance operations differ from one another, presumablybecause the surfaces are covered with different chemical species. Theinterior surfaces bordering the substrate processing region are coatedafter fluorine treatment operation 125 such that the etch rate is stableat a value similar to the first wafer processed in an untreated chamber(the left-most data point in untreated data 310). The interior surfacesare coated after hydrogen treatment operation 225 such that the etchrate is stable at a higher silicon etch rate which roughly matches theasymptotic extension of the etch rates for untreated data 310 after alarge number of wafers is processed. The appropriate preventativemaintenance procedure may be selected based on a variety of processcharacteristics such as magnitude of etch rate, particle performance andetch rate within-wafer uniformity.

Additional process parameters are disclosed in the course of describingan exemplary processing chamber and system.

Exemplary Processing System

Processing chambers that may implement embodiments of the presentinvention may be included within processing platforms such as theCENTURA® and PRODUCER® systems, available from Applied Materials, Inc.of Santa Clara, Calif.

FIG. 4A is a substrate processing chamber 1001 according to embodiments.A remote plasma system 1010 may process a fluorine-containing precursorwhich then travels through a gas inlet assembly 1011. Two distinct gassupply channels are visible within the gas inlet assembly 1011. A firstchannel 1012 carries a gas that passes through the remote plasma system1010 (RPS), while a second channel 1013 bypasses the remote plasmasystem 1010. Either channel may be used for the fluorine-containingprecursor in embodiments. On the other hand, the first channel 1012 maybe used for the process gas and the second channel 1013 may be used fora treatment gas. The lid (or conductive top portion) 1021 and aperforated partition 1053 are shown with an insulating ring 1024 inbetween, which allows a unipolar oscillating voltage to be applied tothe lid 1021 relative to perforated partition 1053. The unipolaroscillating voltage strikes a plasma in chamber plasma region 1020. Theprocess gas may travel through first channel 1012 into chamber plasmaregion 1020 and may be excited by a plasma in chamber plasma region 1020alone or in combination with remote plasma system 1010. If the processgas (the fluorine-containing precursor) flows through second channel1013, then only the chamber plasma region 1020 is used for excitation.The combination of chamber plasma region 1020 and/or remote plasmasystem 1010 may be referred to as a remote plasma region herein. Theperforated partition (also referred to as a showerhead) 1053 separateschamber plasma region 1020 from a substrate processing region 1070beneath showerhead 1053. Showerhead 1053 allows a plasma present inchamber plasma region 1020 to avoid directly exciting gases in substrateprocessing region 1070, while still allowing excited species to travelfrom chamber plasma region 1020 into substrate processing region 1070.

Showerhead 1053 is positioned between chamber plasma region 1020 andsubstrate processing region 1070 and allows plasma effluents (excitedderivatives of precursors or other gases) created within remote plasmasystem 1010 and/or chamber plasma region 1020 to pass through aplurality of through-holes 1056 that traverse the thickness of theplate. The showerhead 1053 also has one or more hollow volumes 1051which can be filled with a precursor in the form of a vapor or gas (suchas the fluorine-containing precursor) and pass through blind-holes 1055into substrate processing region 1070 but not directly into chamberplasma region 1020. Showerhead 1053 is thicker than the length of thesmallest diameter 1050 of the through-holes 1056 in embodiments. Tomaintain a significant concentration of excited species penetrating fromchamber plasma region 1020 to substrate processing region 1070, thelength 1026 of the smallest diameter 1050 of the through-holes may berestricted by forming larger diameter portions of through-holes 1056part way through the showerhead 1053. The length of the smallestdiameter 1050 of the through-holes 1056 may be the same order ofmagnitude as the smallest diameter of the through-holes 1056 or less inembodiments. Showerhead 1053 may be referred to as a dual-channelshowerhead, a dual-zone showerhead, a multi-channel showerhead or amulti-zone showerhead to convey the existence of through-holes andblind-holes for introducing precursors.

Showerhead 1053 may be configured to serve the purpose of an ionsuppressor as shown in FIG. 4A. Alternatively, a separate processingchamber element may be included (not shown) which suppresses the ionconcentration traveling into substrate processing region 1070. Lid 1021and showerhead 1053 may function as a first electrode and secondelectrode, respectively, so that lid 1021 and showerhead 1053 mayreceive different electric voltages. In these configurations, unipolaroscillating electrical power may be applied to lid 1021, showerhead1053, or both. For example, electrical power may be applied to lid 1021while showerhead 1053 (and/or an ion suppressor) is grounded. Thesubstrate processing system may include a unipolar oscillating voltagegenerator that provides electrical power to the lid 1021 or showerhead1053 while the other is grounded. The voltage applied to lid 1021 mayfacilitate a uniform distribution of plasma (i.e., reduce localizedplasma) within chamber plasma region 1020. To enable the formation of aplasma in chamber plasma region 1020, insulating ring 1024 mayelectrically insulate lid 1021 from showerhead 1053. Insulating ring1024 may be made from a ceramic and may have a high breakdown voltage toavoid sparking. Portions of substrate processing chamber 1001 near thecapacitively-coupled plasma components just described may furtherinclude a cooling unit (not shown) that includes one or more coolingfluid channels to cool surfaces exposed to the plasma with a circulatingcoolant (e.g., water).

In the embodiment shown, showerhead 1053 may distribute (viathrough-holes 1056) process gases which contain fluorine, hydrogenand/or plasma effluents of such process gases upon excitation by aplasma in chamber plasma region 1020. In embodiments, the process gasintroduced into the remote plasma system 1010 and/or chamber plasmaregion 1020 may contain fluorine. The process gas may also include acarrier gas such as helium, argon, nitrogen (N₂), etc. Plasma effluentsmay include ionized or neutral derivatives of the process gas and mayalso be referred to herein as radical-fluorine referring to the atomicconstituent of the process gas introduced.

Through-holes 1056 are configured to suppress the migration ofionically-charged species out of the chamber plasma region 1020 whileallowing uncharged neutral or radical species to pass through showerhead1053 into substrate processing region 1070. These uncharged species mayinclude highly reactive species that are transported with less-reactivecarrier gas by through-holes 1056. As noted above, the migration ofionic species by through-holes 1056 may be reduced, and in someinstances completely suppressed. Controlling the amount of ionic speciespassing through showerhead 1053 provides increased control over the gasmixture brought into contact with the underlying wafer substrate, whichin turn increases control of the deposition and/or etch characteristicsof the gas mixture. For example, adjustments in the ion concentration ofthe gas mixture can alter the etch selectivity (e.g., thesilicon:silicon nitride etch rate ratio).

In embodiments, the number of through-holes 1056 may be between about 60and about 2000. Through-holes 1056 may have a variety of shapes but aremost easily made round. The smallest diameter 1050 of through-holes 1056may be between about 0.5 mm and about 20 mm or between about 1 mm andabout 6 mm in embodiments. There is also latitude in choosing thecross-sectional shape of through-holes, which may be made conical,cylindrical or combinations of the two shapes. The number of blind-holes1055 used to introduce unexcited precursors into substrate processingregion 1070 may be between about 100 and about 5000 or between about 500and about 2000 in different embodiments. The diameter of the blind-holes1055 may be between about 0.1 mm and about 2 mm.

Through-holes 1056 may be configured to control the passage of theplasma-activated gas (i.e., the ionic, radical, and/or neutral species)through showerhead 1053. For example, the aspect ratio of the holes(i.e., the hole diameter to length) and/or the geometry of the holes maybe controlled so that the flow of ionically-charged species in theactivated gas passing through showerhead 1053 is reduced. Through-holes1056 in showerhead 1053 may include a tapered portion that faces chamberplasma region 1020, and a cylindrical portion that faces substrateprocessing region 1070. The cylindrical portion may be proportioned anddimensioned to control the flow of ionic species passing into substrateprocessing region 1070. An adjustable electrical bias may also beapplied to showerhead 1053 as an additional means to control the flow ofionic species through showerhead 1053.

Alternatively, through-holes 1056 may have a smaller inner diameter (ID)toward the top surface of showerhead 1053 and a larger ID toward thebottom surface. In addition, the bottom edge of through-holes 1056 maybe chamfered to help evenly distribute the plasma effluents in substrateprocessing region 1070 as the plasma effluents exit the showerhead andpromote even distribution of the plasma effluents and precursor gases.The smaller ID may be placed at a variety of locations alongthrough-holes 1056 and still allow showerhead 1053 to reduce the iondensity within substrate processing region 1070. The reduction in iondensity results from an increase in the number of collisions with wallsprior to entry into substrate processing region 1070. Each collisionincreases the probability that an ion is neutralized by the acquisitionor loss of an electron from the wall. Generally speaking, the smaller IDof through-holes 1056 may be between about 0.2 mm and about 20 mm. Inother embodiments, the smaller ID may be between about 1 mm and 6 mm orbetween about 0.2 mm and about 5 mm. Further, aspect ratios of thethrough-holes 1056 (i.e., the smaller ID to hole length) may beapproximately 1 to 20. The smaller ID of the through-holes may be theminimum ID found along the length of the through-holes. Thecross-sectional shape of through-holes 1056 may be generallycylindrical, conical, or any combination thereof.

FIG. 4B is a bottom view of a showerhead 1053 for use with a processingchamber according to embodiments. Showerhead 1053 corresponds with theshowerhead shown in FIG. 4A. Through-holes 1056 are depicted with alarger inner-diameter (ID) on the bottom of showerhead 1053 and asmaller ID at the top. Blind-holes 1055 are distributed substantiallyevenly over the surface of the showerhead, even amongst thethrough-holes 1056 which helps to provide more even mixing than otherembodiments described herein.

An exemplary patterned substrate may be supported by a pedestal (notshown) within substrate processing region 1070 when fluorine-containingplasma effluents arrive through through-holes 1056 in showerhead 1053.Though substrate processing region 1070 may be equipped to support aplasma for other processes such as curing, no plasma is present duringthe etching of patterned substrate, in embodiments.

A plasma may be ignited either in chamber plasma region 1020 aboveshowerhead 1053 or substrate processing region 1070 below showerhead1053. A plasma is present in chamber plasma region 1020 to produce theradical-fluorine from an inflow of the fluorine-containing precursor. Aunipolar oscillating voltage (shifted or otherwise transformed togenerally confine to one polarity) is applied between the conductive topportion (lid 1021) of the processing chamber and showerhead 1053 toignite a plasma in chamber plasma region 1020 during deposition. Theunipolar oscillating voltage applied to lid 1021 is shifted such to notcenter about the potential of showerhead 1053. A unipolar oscillatingvoltage power supply generates a unipolar oscillating frequency of lessthan or about 1,000 kHz, less than or about 500 kHz, less than or about300 kHz or between 1 kHz and 200 kHz according to embodiments.

The top plasma may be left at low or no power when the bottom plasma inthe substrate processing region 1070 is turned on to either cure a filmor clean the interior surfaces bordering substrate processing region1070. A plasma in substrate processing region 1070 is ignited byapplying the unipolar oscillating voltage between showerhead 1053 andthe pedestal or bottom of the chamber. A cleaning gas may be introducedinto substrate processing region 1070 while the plasma is present.

The pedestal may have a heat exchange channel through which a heatexchange fluid flows to control the temperature of the substrate. Thisconfiguration allows the substrate temperature to be cooled or heated tomaintain relatively low temperatures (from room temperature throughabout 120° C.). The heat exchange fluid may comprise ethylene glycol andwater. The wafer support platter of the pedestal (preferably aluminum,ceramic, or a combination thereof) may also be resistively heated toachieve relatively high temperatures (from about 120° C. through about1100° C.) using an embedded single-loop embedded heater elementconfigured to make two full turns in the form of parallel concentriccircles. An outer portion of the heater element may run adjacent to aperimeter of the support platter, while an inner portion runs on thepath of a concentric circle having a smaller radius. The wiring to theheater element passes through the stem of the pedestal.

The chamber plasma region and/or a region in a remote plasma system maybe referred to as a remote plasma region. In embodiments, the radicalprecursors (e.g. radical-fluorine) are formed in the remote plasmaregion and travel into the substrate processing region where they mayindividually react with chamber walls or the substrate surface. Plasmapower may essentially be applied only to the remote plasma region, inembodiments, to ensure that the radical-fluorine (which may also bereferred to as plasma effluents) are not further excited in thesubstrate processing region.

In embodiments employing a chamber plasma region, the excited plasmaeffluents are generated in a section of the substrate processing regionpartitioned from a deposition region. The deposition region, also knownherein as the substrate processing region, is where the plasma effluentsmix and react to etch the patterned substrate (e.g., a semiconductorwafer). The excited plasma effluents may also be accompanied by inertgases (in the exemplary case, argon). The substrate processing regionmay be described herein as “plasma-free” during etching of thesubstrate. “Plasma-free” does not necessarily mean the region is devoidof plasma. A relatively low concentration of ionized species and freeelectrons created within the remote plasma region do travel throughpores (apertures) in the partition (showerhead/ion suppressor) due tothe shapes and sizes of through-holes 1056. In some embodiments, thereis essentially no concentration of ionized species and free electronswithin the substrate processing region. In embodiments, the electrontemperature may be less than 0.5 eV, less than 0.45 eV, less than 0.4eV, or less than 0.35 eV in substrate processing region 1070 duringexcitation of a remote plasma. The borders of the plasma in the chamberplasma region are hard to define and may encroach upon the substrateprocessing region through the apertures in the showerhead. In the caseof an inductively-coupled plasma, a small amount of ionization may beeffected within the substrate processing region directly. Furthermore, alow intensity plasma may be created in the substrate processing regionwithout eliminating desirable features of the forming film. All causesfor a plasma having much lower intensity ion density than the chamberplasma region (or a remote plasma region, for that matter) during thecreation of the excited plasma effluents do not deviate from the scopeof “plasma-free” as used herein.

The fluorine-containing precursor) may be flowed into chamber plasmaregion 1020 at rates between about 5 sccm and about 500 sccm, betweenabout 10 sccm and about 300 sccm, between about 25 sccm and about 200sccm, between about 50 sccm and about 150 sccm or between about 75 sccmand about 125 sccm in embodiments.

The flow rate of the fluorine-containing precursor into the chamber mayaccount for 0.05% to about 20% by volume of the overall gas mixture; theremainder being carrier gases. The fluorine-containing precursor areflowed into the remote plasma region but the plasma effluents have thesame volumetric flow ratio, in embodiments. A purge or carrier gas maybe initiated into the remote plasma region before that of thefluorine-containing gas to stabilize the pressure within the remoteplasma region.

Plasma power applied to the remote plasma region can be a variety offrequencies or a combination of multiple frequencies. In the exemplaryprocessing system the plasma is provided by unipolar oscillating powerdelivered between lid 1021 and showerhead 1053. The energy is appliedusing a capacitively-coupled plasma unit. The remote plasma source powermay be between about 10 watts and about 3000 watts, between about 20watts and about 2000 watts, between about 30 watts and about 1000 watts,or between about 40 watts and about 500 watts in embodiments.

Substrate processing region 1070 can be maintained at a variety ofpressures during the flow of carrier gases and plasma effluents intosubstrate processing region 1070. The pressure within the substrateprocessing region is below or about 50 Torr, below or about 30 Torr orbelow or about 20 Torr. The pressure may be above or about 0.1 Torr,above or about 0.2 Torr, above or about 0.5 Torr or above or about 1Torr in embodiments. Lower limits on the pressure may be combined withupper limits on the pressure to obtain embodiments.

In one or more embodiments, the substrate processing chamber 1001 can beintegrated into a variety of multi-processing platforms, including theProducer™ GT, Centura™ AP and Endura™ platforms available from AppliedMaterials, Inc. located in Santa Clara, Calif. Such a processingplatform is capable of performing several processing operations withoutbreaking vacuum. Processing chambers that may implement embodiments ofthe present invention may include dielectric etch chambers or a varietyof chemical vapor deposition chambers, among other types of chambers.

Embodiments of the etching systems may be incorporated into largerfabrication systems for producing integrated circuit chips. FIG. 5 showsone such system 1101 of etching, deposition, baking and curing chambersaccording to embodiments. In the figure, a pair of FOUPs (front openingunified pods) 1102 supply substrate substrates (e.g., 300 mm diameterwafers) that are received by robotic arms 1104 and placed into a lowpressure holding areas 1106 before being placed into one of the waferprocessing chambers 1108 a-f. A second robotic arm 1110 may be used totransport the substrate wafers from the low pressure holding areas 1106to the wafer processing chambers 1108 a-f and back. Each waferprocessing chamber 1108 a-f, can be outfitted to perform a number ofsubstrate processing operations including the dry etch processesdescribed herein in addition to cyclical layer deposition (CLD), atomiclayer deposition (ALD), chemical vapor deposition (CVD), physical vapordeposition (PVD), etch, pre-clean, degas, orientation and othersubstrate processes.

The wafer processing chambers 1108 a-f may include one or more systemcomponents for depositing, annealing, curing and/or etching a dielectricfilm on the substrate wafer. In one configuration, two pairs of theprocessing chamber (e.g., 1108 c-d and 1108 e-f) may be used to depositdielectric material on the substrate, and the third pair of processingchambers (e.g., 1108 a-b) may be used to etch the deposited dielectric.In another configuration, all three pairs of chambers (e.g., 1108 a-f)may be configured to etch a dielectric film on the substrate. Any one ormore of the processes described may be carried out on chamber(s)separated from the fabrication system shown in different embodiments.

The substrate processing system is controlled by a system controller. Inan exemplary embodiment, the system controller includes a hard diskdrive, a floppy disk drive and a processor. The processor contains asingle-board computer (SBC), analog and digital input/output boards,interface boards and stepper motor controller boards. Various parts ofCVD system conform to the Versa Modular European (VME) standard whichdefines board, card cage, and connector dimensions and types. The VMEstandard also defines the bus structure as having a 16-bit data bus anda 24-bit address bus.

System controller 1157 is used to control motors, valves, flowcontrollers, power supplies and other functions required to carry outprocess recipes described herein. A gas handling system 1155 may also becontrolled by system controller 1157 to introduce gases to one or all ofthe wafer processing chambers 1108 a-f. System controller 1157 may relyon feedback from optical sensors to determine and adjust the position ofmovable mechanical assemblies in gas handling system 1155 and/or inwafer processing chambers 1108 a-f. Mechanical assemblies may includethe robot, throttle valves and susceptors which are moved by motorsunder the control of system controller 1157.

In an exemplary embodiment, system controller 1157 includes a hard diskdrive (memory), USB ports, a floppy disk drive and a processor. Systemcontroller 1157 includes analog and digital input/output boards,interface boards and stepper motor controller boards. Various parts ofmulti-chamber processing system 1101 which contains substrate processingchamber 1001 are controlled by system controller 1157. The systemcontroller executes system control software in the form of a computerprogram stored on computer-readable medium such as a hard disk, a floppydisk or a flash memory thumb drive. Other types of memory can also beused. The computer program includes sets of instructions that dictatethe timing, mixture of gases, chamber pressure, chamber temperature, RFpower levels, susceptor position, and other parameters of a particularprocess.

A process for etching, depositing or otherwise processing a film on asubstrate or a process for cleaning chamber can be implemented using acomputer program product that is executed by the controller. Thecomputer program code can be written in any conventional computerreadable programming language: for example, 68000 assembly language, C,C++, Pascal, Fortran or others. Suitable program code is entered into asingle file, or multiple files, using a conventional text editor, andstored or embodied in a computer usable medium, such as a memory systemof the computer. If the entered code text is in a high level language,the code is compiled, and the resultant compiler code is then linkedwith an object code of precompiled Microsoft Windows® library routines.To execute the linked, compiled object code the system user invokes theobject code, causing the computer system to load the code in memory. TheCPU then reads and executes the code to perform the tasks identified inthe program.

The interface between a user and the controller may be via atouch-sensitive monitor and may also include a mouse and keyboard. Inone embodiment two monitors are used, one mounted in the clean room wallfor the operators and the other behind the wall for the servicetechnicians. The two monitors may simultaneously display the sameinformation, in which case only one is configured to accept input at atime. To select a particular screen or function, the operator touches adesignated area on the display screen with a finger or the mouse. Thetouched area changes its highlighted color, or a new menu or screen isdisplayed, confirming the operator's selection.

As used herein “substrate” may be a support substrate with or withoutlayers formed thereon. The patterned substrate may be an insulator or asemiconductor of a variety of doping concentrations and profiles andmay, for example, be a semiconductor substrate of the type used in themanufacture of integrated circuits. Exposed “silicon” of the patternedsubstrate is predominantly Si but may include minority concentrations ofother elemental constituents (e.g. nitrogen, oxygen, hydrogen, carbon).Exposed “silicon nitride” of the patterned substrate is predominantlySi₃N₄ but may include minority concentrations of other elementalconstituents (e.g. oxygen, hydrogen, carbon). Exposed “silicon oxide” ofthe patterned substrate is predominantly SiO₂ but may include minorityconcentrations of other elemental constituents (e.g. nitrogen, hydrogen,carbon). In some embodiments, silicon oxide films etched using themethods disclosed herein consist essentially of silicon and oxygen.

The term “precursor” is used to refer to any process gas which takespart in a reaction to either remove material from or deposit materialonto a surface. “Plasma effluents” describe gas exiting from the chamberplasma region and entering the substrate processing region. Plasmaeffluents are in an “excited state” wherein at least some of the gasmolecules are in vibrationally-excited, dissociated and/or ionizedstates. A “radical precursor” is used to describe plasma effluents (agas in an excited state which is exiting a plasma) which participate ina reaction to either remove material from or deposit material on asurface. “Radical-fluorine”) are radical precursors which containfluorine but may contain other elemental constituents. The phrase “inertgas” refers to any gas which does not form chemical bonds when etchingor being incorporated into a film. Exemplary inert gases include noblegases but may include other gases so long as no chemical bonds areformed when (typically) trace amounts are trapped in a film.

The terms “gap” and “trench” are used throughout with no implicationthat the etched geometry has a large horizontal aspect ratio. Viewedfrom above the surface, trenches may appear circular, oval, polygonal,rectangular, or a variety of other shapes. A trench may be in the shapeof a moat around an island of material. The term “via” is used to referto a low aspect ratio trench (as viewed from above) which may or may notbe filled with metal to form a vertical electrical connection. As usedherein, a conformal etch process refers to a generally uniform removalof material on a surface in the same shape as the surface, i.e., thesurface of the etched layer and the pre-etch surface are generallyparallel. A person having ordinary skill in the art will recognize thatthe etched interface likely cannot be 100% conformal and thus the term“generally” allows for acceptable tolerances.

Having disclosed several embodiments, it will be recognized by those ofskill in the art that various modifications, alternative constructions,and equivalents may be used without departing from the spirit of thedisclosed embodiments. Additionally, a number of well known processesand elements have not been described to avoid unnecessarily obscuringthe present invention. Accordingly, the above description should not betaken as limiting the scope of the invention.

Where a range of values is provided, it is understood that eachintervening value, to the tenth of the unit of the lower limit unlessthe context clearly dictates otherwise, between the upper and lowerlimits of that range is also specifically disclosed. Each smaller rangebetween any stated value or intervening value in a stated range and anyother stated or intervening value in that stated range is encompassed.The upper and lower limits of these smaller ranges may independently beincluded or excluded in the range, and each range where either, neitheror both limits are included in the smaller ranges is also encompassedwithin the invention, subject to any specifically excluded limit in thestated range. Where the stated range includes one or both of the limits,ranges excluding either or both of those included limits are alsoincluded.

As used herein and in the appended claims, the singular forms “a”, “an”,and “the” include plural referents unless the context clearly dictatesotherwise. Thus, for example, reference to “a process” includes aplurality of such processes and reference to “the dielectric material”includes reference to one or more dielectric materials and equivalentsthereof known to those skilled in the art, and so forth.

Also, the words “comprise,” “comprising,” “include,” “including,” and“includes” when used in this specification and in the following claimsare intended to specify the presence of stated features, integers,components, or steps, but they do not preclude the presence or additionof one or more other features, integers, components, steps, acts, orgroups.

The invention claimed is:
 1. A method of processing a substrate, themethod comprising: transferring the substrate into a substrateprocessing region of a substrate processing chamber; applying a unipolaroscillating voltage between an electrode and a perforated plate, whereinthe unipolar oscillating voltage, on average, biases the perforatedplate at a positive voltage relative to the electrode; forming a remoteplasma between the electrode and the perforated plate to form plasmaeffluents; flowing the plasma effluents into the substrate processingregion housing the substrate, wherein the plasma effluents flow into thesubstrate processing region through perforations in the perforatedplate; and reacting the plasma effluents with the substrate.
 2. Themethod of claim 1 wherein the unipolar oscillating voltage biases theelectrode relative to the perforated plate at a positive potentialthroughout the operation of reacting the plasma effluents with thesubstrate.
 3. The method of claim 1 wherein the perforated plate isgrounded and the unipolar oscillating voltage biases the electrode at anegative voltage.
 4. The method of claim 1 further comprising:processing a DC voltage by passing the DC voltage through a full bridgeformed from four switching devices to form the unipolar oscillatingvoltage.
 5. The method of claim 1 further comprising: processing abipolar oscillating voltage by adding a DC offset voltage to the bipolaroscillating voltage to form the unipolar oscillating voltage.
 6. Themethod of claim 1 further comprising: processing a bipolar oscillatingvoltage with a diode to remove a positive portion of the bipolaroscillating voltage to form the unipolar oscillating voltage.
 7. Themethod of claim 1 further comprising: sending a bipolar oscillatingvoltage through a diode bridge to form the unipolar oscillating voltage.8. The method of claim 1 wherein the electrode is cup-shaped such that aperpendicular distance from the perforated plate to the electrode isgreater near the center of the perforated plate than a perpendiculardistance from the perforated plate to the electrode near the edge. 9.The method of claim 1 wherein the electrode is nickel plated.
 10. Amethod of etching a substrate, the method comprising: transferring thesubstrate into a substrate processing region of a substrate processingchamber; applying a unipolar oscillating voltage between an electrodeand a perforated plate, wherein the unipolar oscillating voltagenegatively biases the electrode relative to the perforated plate;flowing a halogen-containing precursor into a remote plasma between theelectrode and the perforated plate while forming a remote plasma to formplasma effluents; flowing the plasma effluents into the substrateprocessing region housing the substrate, wherein the plasma effluentsflow into the substrate processing region through perforations in theperforated plate; and etching the substrate with the plasma effluents.11. The method of claim 10 wherein the perforated plate is electricallygrounded during the operation of applying the unipolar oscillatingvoltage to the electrode.
 12. The method of claim 10 wherein theunipolar oscillating voltage biases the electrode at a negativepotential, relative to the perforated plate, throughout the operation ofreacting the plasma effluents with the substrate.
 13. A method ofetching a patterned substrate, the method comprising: transferring thepatterned substrate into a substrate processing region of a substrateprocessing chamber; sending a DC voltage through a full bridge of fourswitching devices to form a unipolar oscillating voltage; applying theunipolar oscillating voltage between a concave electrode and a flatnickel-plated perforated plate, wherein the unipolar oscillating voltagebiases the concave electrode at a negative voltage relative to the flatnickel-plated perforated plate; flowing a fluorine-containing precursorinto a remote plasma between the concave electrode and the flatnickel-plated perforated plate while forming a remote plasma to formplasma effluents; flowing the plasma effluents into the substrateprocessing region housing the patterned substrate, wherein the plasmaeffluents flow into the substrate processing region through perforationsin the flat nickel-plated perforated plate; and selectively etching thepatterned substrate with the plasma effluents.
 14. The method of claim13 wherein the concave electrode comprises a portion shaped like a cone.15. The method of claim 13 wherein the fluorine-containing precursorcomprises one or more of atomic fluorine, diatomic fluorine, borontrifluoride, chlorine trifluoride, nitrogen, trifluoride, perfluorinatedhydrocarbons, sulfur hexafluoride and xenon difluoride.